This invention relates to the fabrications of microminiature devices and, more particularly, to the manufacture of such devices utilizing the technology known as X-ray lithography.
X-ray lithography is a promising technology being actively explored for the manufacture of microminiature devices such as very-large-scale integratedcircuit devices with submicron feature sizes. In this technology, a resist-coated workpiece is exposed to X-rays via a mask that is interposed between a source of X-rays and the workpiece.
The mask is a key component of X-ray technology and is thus the subject of extensive development. Such a mask comprises an X-ray transparent membrane having an X-ray-opaque device pattern defined thereon.
It is critical that the mask membrane exhibit a high degree of dimensional stability. Further, to facilitate mask-to-workpiece alignment, it is advantageous that the mask membrane be optically transparent.
Various materials from which to make the X-ray mask membrane have been suggested. One such promising material is amorphous hydrogenated boron nitride which exhibits excellent X-ray transmissivity relative to competing materials such as silicon or silicon carbide. It has been shown that boron nitride under tension can be fabricated into large-area mask membranes with high X-ray and optical transparencies, low defect densities and excellent mechanical properties. U.S. Pat. No. 4,522,842 issued to H. J. Levinstein et al describes one such hydrogenated boron nitride mask membrane useful for X-ray lithography.
Recently, the effects on hydrogenated boron nitride of radiation damage caused by long-term and/or high-flux X-ray exposures have been investigated. The results of these investigations are reported in an article entitled "Radiation Damage Effects In Boron Nitride Mask Membranes Subjected to X-ray Exposures," by W. A. Johnson, R. A. Levy, D. J. Resnick, T. E. Saunders and A. W. Yanof, Journal Vacuum Science Technology B. Vol. 5, No. 1, January/February 1987, pages 257-261.
As reported in the cited article, radiation exposure can degrade both the optical and mechanical properties, and thus impact on the useful lifetime, of mask membranes made of amorphous hydrogenated boron nitride. Specifically, it is reported in the article that growths will form on exposed boron nitride surfaces irradiated in oxygen-containing ambients; optical transmission of irradiated mask membranes will degrade with cumulative absorbed dose; and, most significantly, the tensile stress of a hydrogenated boron nitride mask membrane will decrease under irradiation and introduce distortions in the device pattern thereon.
In practice, the technology of X-ray lithography is advancing towards higher power X-ray sources and step-and-repeat modes of operation. As this occurs, X-ray mask membranes must tolerance significantly greater radiation doses. But, as indicated, cumulative radiation-induced changes appear to constitute a serious and fundamental wear-out mechanism for masks made of hydrogenated boron nitride. In turn, this threatens to curtail the use of this otherwise attractive material in X-ray lithography.
Accordingly, considerable effectors by workers in the art are being directed at trying to understand the aforestated radiation-induced phenomena and to devise mask membranes for X-ray lithography that do not exhibit the above-described deleterious effects. It was recognized that these efforts if successful would contribute significantly to the realization of better X-ray masks and hence to producing high-quality microminiature devices at a lower cost than heretofore considered possible.